This invention relates to synchronous filtering using a charge sharing structure.
Filtering of analog signals often involves modulation or demodulation in order move signals of interest from one frequency to another. An example relates processing a continuous time signal x(t) to recovery of a signal component with energy in the vicinity of fc (Hertz) by first bandpass filtering the input signal x(t) retaining signal components in the vicinity of fc and then modulating the signal with a periodic signal m(t) with a period 1/fc. Referring to FIG. 1, a filter system 10 includes an analog continuous time bandpass filter 14, for example, implemented using discrete circuit components. The output of the bandpass filter is passed to a mutliplexor 16, which periodically switches between passing (i.e., tracking) the output of the bandpass filter and outputting a zero value. Functionally this clocked multiplexor is equivalent to multiplying by a squarewave signal that alternative between 0.0 and 1.0 with a frequency fc and a duty cycle of 50%. The square wave signal has frequency components at zero frequency (“DC”), frequency fc as well as at the odd harmonics of fc. Therefore, in the frequency domain, the spectrum of the multiplexor 16 is a convolution of the bandpass signal output convolved with the odd harmonics of fc. Therefore, the desired signal in the vicinity of fc in x(t) is modulated to zero frequency, as are the components of the output of the bandpass filter at the odd harmonics of fc. To the extent that the bandpass filter sufficiently attenuated the signal in the vicinity of those harmonics, their impact on the output of the multiplexor 106 is not significant. In this example, the output of the modulator 16 (i.e., the output of the filter system 10) is passed through a lowpass filter and an analog to digital converter 18 for further processing. (Note that in some such implementations, the alternation is between the output of the bandpass filter and an inversion (negative) of the filter output.) The analog lowpass filter filters out odd harmonics of fc introduced by virtue of the multiplexor not implementing a multiplication by an ideal sinusoid. In some such examples, the multiplexor is implemented using Balanced Modulator/Demodulator, part AD630, manufactured by Analog Devices, Inc.
With the bandpass filter 14 implemented in analog components, the phase and magnitude at the desired center frequency fc may be difficult to control. For example, small changes in the actual center frequency of the filter may have relatively large effects on the phase at the desired (as opposed to actual filter) center frequency. Therefore, even if the modulation frequency for the multiplexor is exactly fc, the unpredictable phase response of the filter may make it difficult to achieve coherent processing.
Modulator blocks are used in a variety of other applications at the input and/or output of analog filter blocks, for example, implemented as analog circuits or implemented using discrete time digitized sample signal processors.